Do different culture intervals (2 × 24 hours) after thaw of cleavage stage embryos affect pregnancy rates? A randomized controlled trial.

The aim of the study was to evaluate whether selecting embryos for transfer after prolonged culture after thaw (18-24 h) has better pregnancy rates than selecting embryos for transfer after short culture after thaw (2-5 h). We performed a double-blinded, randomized, controlled trial, evaluating 388 patients submitted to ART treatment who had embryos frozen on day-2 and subsequently transferred. All patients received the same endometrial priming with estradiol valerate followed by vaginal progesterone. Patients were randomized for Frozen embryo transfer 2-5 h after thaw (Group D2) or 18-24 h after thaw (Group D2/D3). The main Outcome Measure was ongoing pregnancy rate (OPR) at 20 weeks' gestation per embryo transfer. A total of 179 patients had embryos transferred 2-5 h after thaw and 209 patients had embryos transferred 18-24 h after thaw. The mean age in group D2 was 36 ± 4.4 and 36 ± 5.4 in group D2/D3. Ongoing pregnancy rate was 28% and 33.5% (p = 0.2) for groups D2 and D2/D3, respectively. These results suggest that increasing the culture time of embryos in one day to improve selection before transfer does not increase ongoing pregnancy rate. CLINICAL TRIAL REGISTRATION NUMBER: NCT03381001.